It is important to find suitable analytical techniques to detect and characterize petroleum products. Several methods have been proposed to provide identification of crude oil, fuel oil and residual distillates, Adlard, E. R., J. Inst. Petroleum, 1972, 58, 63; Baier, R. E., J. Geophys. Res., 1972, 77, 5062; Cole, R. D., J. Inst. Petroleum, 1968, 54, 288; Kawahara, F. K., J. Environ. Sci. Technol., 1969, 3, 150; and Mattson, J. S., Anal. Chem., 1971, 43, 1872. Infrared spectroscopic data analysis has been used to identify the type and source of a large number of petroleum samples, Lynch, P. F. and Brown, C. W., Environ. Sci. Technol., 1973, 13, 1123; Brown, C. W., Lynch, P. F. and Ahmadjian, M., Anal. Chem., 1974, 46, 183; Brown, C. W., Lynch, P. F. and Ahmadjian, M., Appl. Spectro. Rev., 1975, 9, 223; and Brown, C. W. and Lynch, P. F., Anal. Chem., 1975, 48, 191. Most of these methods used bands in the region of 650-1200 cm.sup.-1 to characterize petroleum samples, since crude oils and their various distillates provide unique finger prints in this region, which can be used to identify the source of oil slicks.
The present invention embodies a metal halide fiber optic to measure the spectra of petroleum. The fiber optic serves both as a waveguide for transmitting the signal to and from petroleum in remote locations, and also as an intrinsic part of the sensor element to obtain evanescent wave spectra. Light travels down the core of the fiber with total reflection at each interface of the core and the cladding or surrounding material of lower refractive index. Preferably, the light penetrates the cladding or surrounding material by approximately 0.1.lambda. (.lambda.=wavelength) and sets up a standing evanescent wave in the surrounding material. During this short penetration, the evanescent wave is attenuated at various wavelengths due to vibrational transitions of the chemical groups in the cladding or the surrounding material. Thus, the light is not totally reflected at all wavelengths because of the attenuations. The resulting absorption spectrum is referred to as evanescent wave or attenuated total reflection (ATR) spectrum of the cladding or the surrounding material.
In the preferred embodiment of the invention, a silver halide fiber without a cladding, is placed directly in a petroleum sample to obtain an evanescent spectrum of the petroleum sample.